When a tier possesses only 1 card it is in "primary" position. There are some effects which allow for 2 cards to be set in a tier. When you can play more than 1 card into a tier this referred to as "Doubling Up". To double up you must play an effect/ability that specifically allows more than 1 action in a tier or states to place 2 cards in a tier.
When 2 cards exist in a tier the first to resolve is the "primary" card and the other is the "secondary" card. No player may have more than 2 cards in a single tier at the same time.
"Primary position" does not mean tier 1 and "secondary" position does not mean tier 2. These cards are placed in the same tier, regardless of which tier you choose.
With the Jump card "Extra Action", at at cost of 2 energy you can double up in a tier. This card specifically mentions 2 cards are set in the same tier. This is different from the double up ability of the ship Brutus Magnus.
The Brutus allows a more comprehensive ability to do ANY two tier actions in the same tier. The basic tier actions are (1) drawing a card from the action deck, (2) setting a card from hand face down into a tier, and (3) passing (doing nothing). Any of these actions can be selected for the ability of the Brutus.
For example, you could draw 2 cards for the tier. Or, you could draw a card from the deck and set a card in the same tier. Or, of course, you could perform the same effect allowed by Extra Action.
You can see several cards designed to work with doubling up on the card database. Maximizing action economy is a very effective way to gain the advantage in any combat game. Make the most of the time you have.
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